Processes for the penetration of stable, aqueous polyurethane-polyurea dispersions by the incorporation of hydrophilic centers (so-called internal emulsifiers) into the macromolecular chain of the polymer are already known and are described for example in German Pat. Nos. 1,184,946 and 1,178,586; German Auslegeschrift No. 1,237,306; German Offenlegungsschriften Nos. 1,495,745; 1,595,602; 1,770,068 and 2,019,324; U.S. Pat. Nos. 3,388,087; 3,461,103; 3,480,592; 3,686,108; and 3,756,992; and by Dieterich et al. in Angew. Chem. 82, 53 (1970).
A distinction is to be made between those processes which require the use of organic solvents for the synthesis of the polyurethane/polyureas and the so-called solvent-free processes.
In the former case, high molecular weight polyurethanes are synthesized by polyaddition in organic solution, in other words in the homogeneous phase, before they are dispersed in water. Since the solid substance of the disperse particles is also to a high degree homogeneous, such dispersions give rise to polyurethane films of exceptionally high quality, sufficient, for example, for coating textiles.
This highly developed process, which includes both anionic and cationic and non-ionic emulsifier segments, has the disadvantage that organic solvents need to be distilled off and rectified by an expensive process. Connected with this is a poor volume/time yield in the manufacturing process. Moreover, the use of organic solvent as reaction medium increases the risk of explosion and fire in the production plant.
Another disadvantage of this process is that the chain lengthened, high molecular weight polyurethane-polyureas must be readily soluble in the organic solvent (e.g. acetone) so that a homogeneous, highly concentrated, but not too viscous, organic solution is obtained before dispersion in water. This condition is frequently not fulfilled in polymers which are required to yield exceptionally hard, high-melting and solvent-resistant coatings such as, for example, polyurethane-polyureas based on diphenyl methane diisocyanates which contain a high proportion of urea groups.
Among the known solvent-free methods of preparing polyurethane dispersions, there should also be mentioned the so-called melt dispersion process as described in German Offenlegungsschrift No. 1,770,068; U.S. Pat. No. 3,756,992 and by D. Dieterich and H. Reiff in Angew. Makromol. Chem. 76, 85 (1972).
In this process, an oligourethane which has been modified with ionic groups and contains acylated amino end groups is converted by means of formaldehyde into the corresponding oligourethane containing methylol end groups attached to acylated amino groups, and this oligourethane is then chain lengthened by a heat treatment which effects condensation of the reactive methylol end groups. This chain lengthening reaction may be carried out in the presence of water so that an aqueous dispersion of a polyurethane is directly obtained. The process is particularly suitable for the preparation of cationically-modified polyurethanes or the preparation of polyurethanes containing anionic carboxylate groups. The required combination of an isocyanate polyaddition reaction with the aforesaid chain lengthening reaction by way of polycondensable methylol groups which are attached to acyl amino end groups is a more complicated procedure than the usual isocyanate polyaddition by the prepolymer process in which prepolymers containing isocyanate groups are reacted with conventional chain lengthening agents such as water or diamines. This added complication in the process according to German Offenlegungsschrift No. 1,770,068 and U.S. Pat. No. 3,756,992 could be justified on the grounds that the process for the first time allowed polyurethane dispersions to be produced without the aid of special stirrers and without the aid of emulsifiers or solvents.
Another process described in German Offenlegungsschrift No. 2,446,440 and U.S. Pat. No. 4,108,814 concerns the solvent-free preparation of polyurethanes in aqueous dispersion. In this process, a prepolymer containing sulphonate groups and isocyanate groups is reacted with a difunctional isocyanate-reactive compound during the dispersion process, i.e. after dissolving the chain lengthening agent in the water of dispersion. it is also possible to add the chain lengthening agent after dispersion of the prepolymer in water.
The characteristic feature of this process is that the synthesis to a high molecular weight polyurethane urea takes place in a disperse, i.e. heterogeneous phase. The chain lengthening agent, i.e. diamine or water, is in the liquid phase and only penetrates to the center of the dispersed particles over a certain period of time. It is clear that the chain lengthening agent, i.e. the diamine dissolved in water, initially reacts in the outer regions of the dispersed particle. The concentration of chain lengthening agent, therefore, decreases progressively towards the center of the particle.
The solid particles of the dispersions obtained by this process are, therefore, not homogeneous, and the resulting polyurethane ureas have less mechanical strength than products produced in homogeneous solution and in some cases also have less resistance to hydrolysis.
In addition, it is virtually impossible to prepare highly concentrated dispersions, with a solids content of more than about 40% by the aforesaid process because at relatively high solids concentrations, the individual dispersed particles are joined together into larger aggregates by the chain lengthening agent which is situated mainly between the particles. This results in very high viscosities which are unsuitable for processing and the aqueous dispersions may even become pasty.
It was, therefore, an object of the present invention to provide a solvent-free or low solvent process by which polyurethanes with improved properties could be obtained in the aqueous phase, and the improvement was mainly to be achieved by the fact that the high molecular weight solid substances distributed in the liquid aqueous medium should be obtained by polyaddition in, as far as possible, a homogeneous phase.
It has now surprisingly been found that exceptionally high quality polyurethanes can be obtained in aqueous dispersions or solution if prepolymers which have isocyanate end groups and contain a hydrophilic group and/or an external emulsifier are mixed, before their dispersion in water, with an at least partially blocked chain lengthening agent which forms hydrazine under the hydrolytic influence of water, and the resulting mixture is then mixed with water. Another surprising observation is that polyurethanes which are substantially resistant to oxidation and light are obtained by this process even when isocyanate prepolymers which have aromatically bound isocyanate groups are used.
The compounds used in the process according to the invention which form hydrazine under the hydrolytic influence of water are azines and/or hydrazones. The reaction of isocyanates with azines has so far been disclosed only to the extent that it is known to give rise to Diels-Alder-type adducts at temperatures above about 100.degree. C. as described in German Auslegeschrift No. 1,044,405 and by J. R. Bailey et al. in J. Amer. Chem. Soc. 39, 279, 1322 (1917). In the present invention, on the other hand, the reactants are reacted together in the presence of water at temperatures at which azines are inert towards isocyanates in the absence of water. It may, therefore, be assumed that a hydrolytic decomposition of the azine and hydrazone groups takes place with the aid of the isocyanates; this is surprising in the view of the fact that azines and hydrazones are normally hydrolyzed only in an acid medium as described by E. C. Gilbert in J. Amer. Chem. Soc. 51, 3394 (1929).